Strange Normal Baking
Courtesy of Lemon!

Distorted bakes

This resolved the issue. I applied an edge split as per the post, but am still unable to really understand WHY that worked. There were no hidden or squashed faces, the face in question is planar (scale y 0 and apply scale changed nothing) and yet it still baked as if it weren't. Odd for sure, but at least there's the solution.

Thanks again!